1. Field of the Invention
The present invention relates to a sintered annular nuclear fuel pellet and, more particularly, to a method for fabricating a sintered annular nuclear fuel pellet without performing an inner side grinding processing so that the sintered annular nuclear fuel pellet can be used for a dual-cooling nuclear fuel rod that heat transfer simultaneously occurs at the inner and the outer claddings.
2. Description of the Related Art
In general, a uranium dioxide (UO2) pellet is the most commonly used nuclear fuel of a commercial reactor. A UO2 pellet contains a certain amount (e.g., 1 wt % to 5 wt %) of U235 and, while it is in use in a nuclear reactor, U235 of the UO2 pellet reacts with a neutron, generating nuclear fission energy. The pellet (i.e., the sintered pellet) of a light-water reactor nuclear fuel has a cylindrical shape (e.g., having a diameter of about 8 mm and a length of about 10 mm). The centers of the upper and lower surfaces of the cylinder are dished up and corners of the upper and lower surfaces are chamfered to have a flat chamfer.
In general, the sintered nuclear fuel pellet is used in cylindrical rod form in such a manner that it is charged in a zirconium alloy cladding tube having a certain length (e.g., about 4 m) in the commercial nuclear reactor. Such a commercial nuclear fuel rod is limited in its performance in terms of temperature and heat flux.
The UO2 pellet (i.e., the sintered pellet) has many advantages as a nuclear fuel, but its thermal conductivity is low, compared with a metal or nitride nuclear fuel, failing to quickly transfer heat generated according to nuclear fission to cooling water, and the pellet therefore has a much higher temperature than that of the cooling water during operations. For example, the cooling water has a temperature ranging from 320 degrees Celsius to 340 degrees Celsius, and the pellet has the highest temperature at its center and the lowest temperature at its surface. The temperature at the center of a pellet of a normally burned nuclear fuel rod ranges from 1,000 degrees Celsius to 1,500 degrees Celsius.
The pellet remaining at a high temperature results in an encroachment of a margin for safety in the occurrence of various design basis nuclear reactor accidents. For example, in the occurrence of a loss of coolant accident, the safety margin becomes smaller, as the temperature of the nuclear fuel immediately before the accident is high. Also, when the heat flux of the nuclear fuel rod increases, a departure of nucleate boiling may occur. The occurrence of the departure of nucleate boiling leads to a formation of an air bubble curtain on the surface of the cladding tube, severely degrading a heat transmission to potentially damage the nuclear fuel rod.
In an effort to solve the problem, an annular nuclear fuel rod (U.S. Pat. No. 3,928,132 entitled ‘Annular fuel element for high temperature reactor’ by Roko Bujas in 1975) was proposed, which includes an outer cladding tube 11, an inner cladding tube 12 coaxially disposed with the outer cladding tube 11 and having a diameter smaller than that of the outer cladding tube 11, and an annular pellet 15 charged between the outer cladding tube 11 and the inner cladding tube 12 as illustrated in FIG. 1 and FIG. 2.
The related art annular nuclear fuel rod 10 allows a coolant to additionally flow along the center having the highest temperature in the cylindrical commercial nuclear fuel rod, so the average temperature of the nuclear fuel rod can be significantly reduced. In addition, because the heat transfer area per nuclear fuel rod is drastically increased to reduce heat flux, so the thermal margin can be improved.
However, heat generated from the annular pellet of the related art annular nuclear fuel rod is transferred to the coolant through the both sides of the inner cladding tube and the outer cladding tube, so if a large amount of heat is transferred to one side, heat transferred to the other side is reduced by as much. The amount of generated heat transferred via either of the inner and outer cladding tubes has a connection with the thermal resistance of both directions, so a larger amount of heat is distributed to the cladding tube having a smaller thermal resistance, causing a problem in that the heat flux of one cladding tube becomes higher than that of the other cladding tube.
About a half of the thermal resistance present in the annular nuclear fuel rod is taken up by the thermal resistance of gaps existing between the pellet and the inner and outer cladding tubes, and in this case, the thermal resistance of the gap is proportional to the size of the gap.
In order to reduce the thermal resistance of the gaps de and di between the annular pellet 15 and the cladding tubes 11 and 12 after fabrication, the gaps are set to be as small as possible within a fabrication range (e.g., 50 μm to 100 μm). Recently, a reduction of the internal gap to below 30 μm has been proposed as a solution to the asymmetry of the heat flux.
Thus, in order to obtain a desired gap size, accurate adjusting of the dimensions of the inner and outer diameters of the annular pellet and precise controlling of the dimension tolerance are crucial in terms of the fabrication of the annular pellet.
In the process of manufacturing the commercial nuclear fuel pellet, a nuclear fuel powder or granule is charged in a mold and then pressed by pressing a vertical molding punch through double acting uniaxial pressing to fabricate a green compact (or a green body), and then, the green body is sintered. The green body fabricated through the double acting uniaxial pressing is sintered to be deformed such that the diameter of a central portion of the green body is smaller than the diameters of the upper and lower portions of the green body, for example, in a shape such as a double-headed drum with a narrow waist in the middle or an hourglass. Thus, the green body undergoes a centerless grinding process in order to have a uniform diameter along length of the pellet.
Compacting process variables affecting the dimension and shape of the sintered pellet include a variation in a green density among various green bodies due to the difference in the amount of powder introduced into the mold during an auto-molding process, a non-uniform green density distribution in a single green body caused by frictional contact on the wall of the molding frame, and the like.
First, as for the variation of the dimension of a pellet due to the difference in green densities among various green bodies, R. M. German (Powder Metallurgy 2004, Vol. 47, No. 2 pp 157-160) revealed that, provided the pressing conditions and sintering conditions are the same, when the dimension of a green body is the same, a green density can be represented by the weight of the green body and there is such a relationship as expressed by Equation (1) shown below between variations of the weight of the green body and the dimension of the sintered pellet.
                                          green            ⁢                                                  ⁢            mass            ⁢                                                  ⁢            variation                                mean            ⁢                                                  ⁢            mass                          ≤                  3          ⁢                      tolerance                          mean              ⁢                                                          ⁢              size                                                          [                  Equation          ⁢                                          ⁢                      (            1            )                          ]            
Namely, it means that, in order to obtain a dimension tolerance of ±0.2 percent between sintered bodies, the variation of the weight of the green body needs to be adjusted to be ±0.6 percent or smaller. However, because there is the potential for another process variable influence in an actual process, the variation of the weight of the green body needs to be controlled more minutely. For example, in the above-mentioned document, the variation of the weight of the green body is adjusted to be 0.2 percent or smaller in the uniaxial pressing process to maintain the dimension tolerance of the sintered body within the range from ±0.14 percent to ±0.20 percent.
Next, the non-uniformity of the green density in a single green body causes a sintering deformation in the shape of a double-headed drum with a narrow waist in the middle or an hourglass. The difference in the green density in the green body causes a difference in the sintering shrinkage of each part of the green body during sintering, triggering deformation and even cracking in a worst case scenario.
FIG. 3 is a schematic view showing a green density distribution within the green body and the shape of a sintered pellet (pellet) according to a pressing direction.
With reference to FIG. 3, a molding device 30 includes a molding frame 32 and an upper punch 31a and a lower punch 31b disposed at upper and lower portions of the molding frame 32. A non-uniform density distribution caused by pressing results from a friction (F1) between powder 25 and the molding frame 32 and a friction (F2) between powder grains 25.
The pressure applied to the surface of the powder 25 filling the molding frame 32 by the pressing punches 31a and 31b is lost due to friction. Thus, in the powder away from the punched surface, the actual working force is reduced compared to the applied pressure. The less-compressed area has a lower green density than that of an area to which a higher pressure is applied. The low-density area is incompletely densified or greatly shrunken compared with other neighboring areas.
A cylindrical-type sintered pellet can obtain precise dimension tolerance with a certain outer diameter through centerless grinding, but in the case of a sintered annular pellet, both the outer surface and inner surface would be deformed, so both inner and outer diameters would require grinding. General centerless grinding can resolve only the tolerance of an outer diameter and an outer diameter dimension.
In order to adjust the tolerance with an inner diameter dimension, the inner side of the sintered pellet needs to be ground. For the inner side grinding, precise grinding using a diamond wheel or sandblasting process may be performed. In this respect, however, because the inner diameter distribution of the sintered annular pellet is different for each sintered pellet, in case of the diamond wheel grinding, the sintered annular pellets must be held one by one and ground, unlike centerless grinding, resulting in degradation of productivity.
In general, inferior or defective products or grinding residues or remnants, namely, those containing high-priced enriched uranium generated from the nuclear fuel fabrication process, are powdered through an oxidation process or the like so as to be recycled. However, in case of sandblasting, uranium and sand are mixed with the grinding residues, making it difficult to recycle the costly uranium content. Thus, a problem arises in separating the uranium from the grinding residues and controlling the impurity contents.